(HealthDay News) -- Truckers and others who are routinely exposed to diesel fumes while on the job might face a greater chance of developing amyotrophic lateral sclerosis (ALS), a new study suggests.

The increased risk hit a high of 40 percent when compared against men with no such exposure, said study author Aisha Dickerson. She's a postdoctoral research fellow with the Harvard T.H. Chan School of Public Health in Boston.

"The strongest association we saw was for occupations that were held at least for 10 years prior to their ALS diagnosis," Dickerson said. "Someone could have been exposed years earlier, before they showed any symptoms of ALS, but the damage would have been done long ago."

Jobs with a lot of diesel exhaust exposure include truck drivers, police officers, shipyard hands, construction workers, farm laborers and tool operators, as well as many people who work in an industrial setting, Dickerson said.

ALS, also known as Lou Gehrig's disease, is a neurodegenerative condition in which the nerve cells responsible for controlling muscle movement wither and die. Patients eventually lose their strength and their ability to walk, move, speak, eat and even breathe. There is no cure, according to the U.S. National Institutes of Health.

Prior studies have suggested a higher risk of ALS in specific jobs that are commonly exposed to diesel fumes, such as truck and bus drivers, construction workers and military personnel, Dickerson said.

To further test this association, Dickerson and her colleagues reviewed three decades of records from the Danish National Patient Registry.

The research team identified 1,639 people diagnosed with ALS between 1982 and 2013, and calculated their estimated diesel exhaust exposure based on their employment history.

The researchers then compared each ALS patient against 100 other people of the same age and sex who did not have ALS.

Any exposure to diesel exhaust at all increased a worker's odds of developing ALS by about 20 percent, the research team found. Further, the more exposure workers got, the more their ALS risk increased.

This risk increase was only seen in men, however, and the study did not prove that diesel exhaust caused ALS.

Any of these compounds, on their own or in combination, could be contributing to the death of nerve cells that results in ALS, Dickerson said. Further research is needed to figure out which particular diesel fume compounds carry the risk.

It's possible that the toxins in diesel fuel are affecting people's genetics in ways that spur on ALS, said Dr. Anthony Geraci, director of the neuromuscular center at Northwell Health in Manhasset, N.Y.

But at least 20 genes have been linked to ALS, and "this clearly is a genetic disorder in a large percentage of individuals with ALS," Geraci added.

"Perhaps those jobs and diesel exhaust and whatever might be in the diesel is hastening a disease that already may be genetically programmed in the individual," Geraci said.

But, Dickerson explained, because this was not a controlled experiment, researchers cannot rule out the possibility of some other explanation for why these occupations might be tied to ALS.

She noted that these sorts of jobs have a high risk of physical injury and exposure to other dangerous chemicals, either of which could be linked to ALS. For example, farmers often handle pesticides and construction workers deal with solvents.

Further research is needed to more definitively link diesel fumes and ALS, Dickerson said.

"I think the next step for future studies would be to look at diesel exhaust in the general population, via proximity to freeways and industrial facilities and things like that," Dickerson suggested.

The findings are to be presented at the American Academy of Neurology's annual meeting, scheduled for April 21 to 27 in Los Angeles. Research presented at meetings should be considered preliminary until published in a peer-reviewed journal.

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